Oriented strand board (“OSB”) is commercially available. OSB material generally is formed of multiple layers of wood “flakes” or “strands” bonded together by a resin binder under heat and compression to provide a unitary board structure. The flakes are made by cutting logs into thin slices with a knife edge oriented parallel to the length of a debarked log. The cut flakes are broken into narrow strands generally having lengths oriented parallel to the wood grain that are larger than the strand widths.
In one common fabrication of oriented strand board, the flakes generally are first dried to remove water, and are then coated in a blender with a thin layer of binder and sizing agent. The coated flakes are then spread on a conveyor belt to provide a surface ply or layer having flakes oriented generally in line with the conveyor belt, then one or more plies that will form an interior ply or plies of the finished board is (are) deposited on the surface ply such that the one or more plies is (are) oriented generally perpendicular to the conveyor belt. Then, another surface ply having flakes oriented generally in line with the conveyor belt is deposited over the intervening one or more plies having flakes oriented generally perpendicular to the conveyor belt. The resulting structure includes plies having flakes oriented generally perpendicular to a neighboring ply insofar, such as for each surface ply and the adjoining interior ply. The layers of oriented “strands” or “flakes” are finally exposed to heat and pressure to bond the strands and binder together to form a consolidated board structure. Other variations on this basic manufacturing scheme also are known, such as, for example, shown in U.S. Pat. No. 6,479,127 B1. The resulting product is then cut to size and shipped. Typically, the resin and sizing agent comprise less than 10% by weight of the oriented strand board.
The manufacturing control that can be achieved in lignocellulosic composites is strongly influenced by the quality of raw materials used. Plywood products typically use peeled veneers to make laminated structural composites, such that less wood compression is required to achieve the engineered strength design value. In general, the wood compressive ratio of plywood is in a range from 1.05 to 1.15, which means that the density of plywood is normally 5% to 15% higher than raw wood materials. For example, the density of plywood made of southern pine will be around 35 to 38 pounds per cubic feet (pcf).
In contrast, oriented strand boards (OSB) are mainly composed of much shorter wood strands or flakes, polymeric binders, and water repellent agents. Technically, to meet the strength requirement, a much higher compressive ratio is needed to permit OSB to have equivalent strength performance as plywood. The preferred compressive ratio for OSB is in a range from 1.15 to 1.30. The current density range for making commercial southern pine OSB is generally around 42 to 45 pounds per cubic feet (pcf) to meet the mechanical and structural requirements for its applications.
Southern pine, which also is often referred to as Southern Yellow Pine, typically is considered as the group of pines including Longleaf pine (Pinus palustris Mill.), Shortleaf pine (P. echinata mill.), Loblolly pine (P. Taeda L.), and Slash pine (P. elliottii Engelm.)
There is an increased use of OSB made of southern pine to replace traditional plywood in the building and construction industry, especially in residential construction. The commercial OSB products are widely used for the roof-decking, wall-sheathing, and sub-flooring applications, concrete forming, among other uses. Some commercial OSB products outperform plywood in term of dimensional stability and mechanical strength performance. The bending strength of OSB can be, for example, 10–20% more than the commodity plywood with less warping and equivalent thickness swelling to plywood. Bending strength is one of the most important engineering parameters in determining the engineering design value of boards to be used in building construction.
In comparing various attributes of OSB with plywood, one of the OSB drawbacks in term of applications is that the weight of current commercial OSB is often about 10–20% heavier than that of similarly-dimensioned panels of plywood. The higher OSB weight relative to plywood is a concern among builders and other users of the products, especially where the product must be transported and handled by hand, such as on roofs and in other construction sites, and so forth. Consequently, a lighter weight OSB that maintains requisite strength performance would be very desirable to builders and others doing construction and the like with board materials.
However, in general, when the density of composite lignocellulosic materials is reduced, the mechanical properties also tend to be reduced simultaneously. Thus, a need has existed for a technique for reducing OSB density or weight for a given panel size without causing a concurrent loss in useful OSB mechanical properties.
Efforts have been made in the past to address the above density and strength concerns in composite boarding. For example, U.S. Pat. No. 5,554,429 to Iwata et al. describes a method for using a foaming binder in a wood board formulation that is a mixture of a foaming resin and a non-foaming resin at a ratio within a range of 4:1 to 1:4. The manufactured OSB is described as having a particular resistance to moisture content with low density and high strength. However, the use of specialty foaming binders in place of normally used consolidation resins can increase manufacturing cost and process and product quality control issues and complexity for board processing.
Another consideration in the fabrication of wood flake mats in commercial oriented strand board is the common practice of discontinuously bonding strands together to form the mats such that significant void spaces remain in the product. These voids can significantly impact the process control and ultimate mechanical properties of the final mat product. The size and shape of these voids that arise in flake boards comprised of discontinuously bonded lignocellulosic wood strands and the like affects the horizontal and vertical density distributions within the panel, as well as the mechanical properties, which is significantly affected by the distribution of voids. Many of these voids can be visually observed in the flake board product. Consequently, flake boards such as OSB are typically composite structures in which the wood component thereof typically constitutes the primary component thereof and which include voids or air spaces.
Various low-density filler materials have been used to reduce the density of fiber-reinforced composite materials or syntactic foam composite materials. Hollow microspheres are a popular low-density filler material. They generally can be hollow glass microspheres, hollow ceramic microspheres, hollow polymeric microspheres, or perlite. The development of microsphere technology has been very active in recent years.
Microsphere materials have been used in various applications including fiber reinforced plastic compounds for bathwares, marine substance, patching including compounds for plywood and gypsum wall boards, modeling clays for art and architectures, building materials for lightweight and reinforcement of gypsum boards, sound attenuation, explosives, reinforced epoxy resin foams, textile printing inks, paper additives, adhesive and sealant fillers, lightweight concrete, wire cables, cosmetics, pharmaceuticals for drug delivery, aerospace syntactic foam panels, deep submarine vehicles, machine tooling materials, and flow line insulation materials for oil drilling, and so forth.
U.S. Pat. No. 6,171,688 B1 to Zheng et al. describes syntactic foams comprising a fiber-reinforced composite including a polymer matrix, polymer microspheres, glass microspheres, dried natural fibers, and wood flour. The '688 patent states that in most cases the materials are mixed to a puttylike state or to a state in which the material can just be cast. The examples in the '688 patent describe polyester resin as the predominant component and kenaf fiber as a non-predominant component in all formulations used to make board. The dried natural fibers are described as being plant fibers, such as kenaf fibers, having hairy projections along their length and a width dimension of between 40 to 60 microns (0.04 to 0.06 mm) and lengths between about 2.5 cm.
Kenaf fiber is a natural lignocellulosic plant material with an annual growth season, which after being oven-dried to less than 2% moisture content, is directly mixed with other composite ingredients. No dimensional reduction is typically required for kenaf fibers before using them in the syntactic foam formulations. Kenaf-fiber reinforced composites are known that are used as interior decorative panels in non-structural applications where board strength is not an important design consideration.
There is a need for improvements in the flake board technology in particular that would permit weight reductions, and alleviate the above-discussed void structure related problem associated with OSB composite board structures and the like, without making compromises in board strength.